1. Technical Field
The present invention relates to a driving circuit for a power transistor wherein a base current has a given function of that of the collector.
2. Description of the Related Art
A common power transistor circuit has a power NPN transistor, or Darlington pair, with an inductor as a load on the collector of the power transistor. A driving circuit supplies an input to the base of the power transistor so that, for a voltage value of zero at the input of the driving circuit, the collector current of the power transistor is zero. When the voltage across the input of the driving circuit goes high, the current on the collector of the power transistor begins to rise in a linear fashion.
Normally, the current necessary to drive the base input of the power transistor is equal to the maximum value of base current beginning immediately when the voltage across the input of the driving device goes high. This maximum value of base current is that value which is suitable for driving the power transistor to its maximum collector current value. Such a driving technique, however, requires unnecessary base current and dissipates extra power. This occurs because it is not necessary to have a maximum value of the base current as soon as the voltage across the input of the driving circuit goes high.
As described in U.S. Pat. No. 5,027,004 issued Jan. 25, 1991, an alternative method is provided to that just described. This alternative method provides for driving the power transistor by using a base current for the power transistor proportional to its collector current. In this technique, a minimum value for the gain of the power transistor is assumed. With the exclusion of the initial switching, the base current of the power transistor is forced to be equal to the ratio between its present collector current and the minimum, presumed constant, gain.
This latter type of drive provides advantages with respect to that previously described, but it is not optimized because the gain of the power transistor is not actually constant as the collector current varies. In addition, the gain of the power transistor changes as the junction temperature varies. In addition, the circuit that is used to accomplish the improved technique is fairly complex. This drive circuit must (1) detect the base current of the power transistor, (2) detect the emitter current of the power transistor, and (3) provide a current amplifier to compare the detected base and emitter current values. The current amplifier generates an output for a control circuit of the power transistor.
The control circuit thus operates on the base current of the power transistor, so as to make it proportional to the collector current. Since this is a feedback circuit, the driving circuit has the disadvantage that it must be frequency compensated, which can give rise to instability.